A paste vinyl chloride resin, which is a general-purpose resin globally the most widely used in household and industrial applications, is prepared by a common method such as emulsion polymerization, micro-suspension polymerization or seed emulsion polymerization.
In accordance with seed emulsion polymerization, the paste vinyl chloride resin is prepared by adding two types of seeds having different mean particle sizes in an early stage of polymerization and growing a vinyl chloride monomer while reacting the vinyl chloride monomer with the seeds to prepare final latex particles.
Of these two types of seeds, the first seed is prepared by adding a vinyl chloride monomer, an emulsifier and a fat-soluble polymerization initiator, homogenizing the components using a rotor-stator type homogenizer pump and polymerizing the resulting substance. The second seed is prepared by emulsion polymerization. The first seed comprises the fat-soluble polymerization initiator in particles thereof and thus inherently has reaction sites. Accordingly, it is necessary that a suitable amount of initiator be left in particles after completion of polymerization through addition of excess initiator during polymerization of the first seed so as to facilitate polymerization initiation during seed emulsion polymerization.
In general, polymerization temperature is adjusted to a low level of 50° C. or less and an initiator having long half-life such as lauryl peroxide (LPO) is used so as to allow a predetermined amount of initiator to remain un-decomposed in particles of the first seed. The particle size of the first seed, and the type or content of remaining initiator are factors greatly affecting polymerization reactivity. Accordingly, there is a need for methods effectively controlling these factors to improve polymerization reactivity.
A variety of additives, in addition to the monomer and the emulsifier, are used for polymerization to prepare vinyl chloride resins according to desired application and physical properties. An ionic or non-ionic emulsifier may be added to improve latex stability, or an acid or base may be used to control hydrogen ion level. Among the ionic and non-ionic emulsifiers, substances used as the non-ionic emulsifier are considerably varied and results change according to the type of non-ionic emulsifier. Accordingly, selection of a substance suitable for the desired application is considerably essential and the desired properties can be easily and simply obtained.